Echo is always present to some extent in all voice networks. For echo to be a problem, which
three conditions must exist? (Choose three.)

A.
analog leakage between analog Tx and Rx paths

B.
sufficient echo amplitude to be perceived as annoying

C.
sufficient power from the talker’s side to cause listener echo

D.
sufficient delay in echo return for echo to be perceived as annoying

E.
an analog two-wire to four-wire hybrid operating below 600 ohm impedance

Explanation:
Echo is perceived as annoying when all of these conditions are true:
http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080149a1f.shtml